1. Field of the Invention
This invention relates in general to the preparation of water soluble salts of carboxyalkyloxy succinic acids and in particular to the preparation of a highly pure form of said salts having low contamination with alkaline earth metals and related sequesterable substances.
2. Description of the Art
It is known that water soluble salts of carboxyalkyloxy succinic acids are useful detergent builder materials. Such is disclosed in U.S. Pat. No. 3,635,830. The preparation of such materials is described in U.S. Pat. No. 3,914,297; however, the preparation of the materials in a form as low in alkaline earth metals and related substances as is desirable is difficult. Especially is it evident that not only does the prior art fail to teach in regard to minimizing contamination by alkaline earth metals and related substances but the prior art does not even recognize the residual contamination problem.
It is known that the typical water soluble carboxymethyloxy succinic acid (CMOS) salts are readily produced by reacting salts of glycolic acid and maleic acid in an aqueous medium in the presence of enough zinc or alkaline earth metal hydroxide, typically calcium hydroxide, to produce a salt system having a pH of at least about 8 and preferably higher than about 11, e.g. 12.1, when measured at 25.degree. C. The reaction is preferably conducted at reflux temperature at about atmospheric pressure for from about 1 to about 6 hours. Typically, the calcium salt system thus obtained is reacted with an alkali metal carbonate such as sodium or potassium carbonate at a moderate temperature of about 60.degree. C., for example, and filtered directly or after cooling to room temperature of about 25.degree. to 30.degree. C. to remove suspended zinc or alkaline earth metal carbonate yielding an aqueous solution containing trisodium CMOS salts or tripotassium CMOS salts as appropriate. Such solutions can also be produced in other ways such as by converting the zinc or alkaline earth metal salt into acid by ion exchange treatment with mineral acid and reacting the CMOS with an organic or inorganic base to produce other salts typified herein such as the ammonium or alkanol ammonium salts.